Seismic monitoring is known as a method with an observation horizon that penetrates far deeper into a hydrocarbon reservoir than any other method employed in the oilfield industry. It has been proposed to exploit the reach of seismic methods for the purpose of reservoir monitoring.
In conventional seismic monitoring a seismic source, such as airguns, vibrators or explosives are activated and generate sufficient acoustic energy to penetrate the earth. Reflected or refracted parts of this energy are then recorded by seismic receivers such as hydrophones and geophones.
In microseismic monitoring the seismic energy is generated through so-called local microseismic events either naturally occurring in the formation or caused by human activity or intervention. The events include seismic events caused by fracturing operations to be described in more detail below, by very small sources injected for example with wellbore fluids, or background events illuminating the area of interest. Those variants of the microseismic methods which lack an actively controlled seismic source are sometimes also referred to as passive seismic monitoring. For the purpose of the present invention, microseismic shall include all of the above described variants.
Referring now in more detail to hydraulic fracturing operations, it is known that production or storage capacity of underground reservoirs can be improved using a procedure known as hydraulic fracturing. Hydraulic fracturing operations are for example commonly performed in formations where oil or gas cannot be easily or economically extracted from the earth from drilled and perforated wellbores alone.
These operations include the steps of pumping through a borehole large amounts of fluid to induce cracks in the earth, thereby creating pathways via which the oil and gas can flow more readily than prior to the fracturing. After a crack is generated, sand or some other proppant material is commonly injected into the crack, such that a crack is kept open even after release of the applied pressure. The particulate proppant provides a conductive pathway for the oil and gas to flow through the newly formed fracture into the main wellbore.
The hydraulic fracturing processes cannot be readily observed, since they are typically thousands of feet or meters below the surface of the earth. Therefore, members of the oil and gas industry have sought diagnostic methods to tell where the fractures are, how big the fractures are, how far they go and how high they grow. As mentioned above, one method of observing fracturing operations has been found in the use of microseismic monitoring.
Apart from the problem of detecting the often faint microseismic events as such, the interpretation of microseismic signals is difficult as often neither the source location nor the source signature or characteristics are known prior to a processing of the measurements. However knowledge of these parameters is important to deduce further reservoir parameters knowledge of which would improve reservoir control. If for example the precise location and the moment tensor of the source or event which caused a seismic wave are required for the further processing and interpretation of the recorded signals or data, then such parameters have to be inferred from the recordings.
Details of known microseismic monitoring methods can be found for example in for example the following publications:                Maxwell S. C., Urbancic T. I., Falls S. D., Zinno R.: “Real-time microseismic mapping of hydraulic fractures in Carthage”, Texas, 70th Annual International Meeting, SEG, Expanded Abstracts, 1449-1452 (2000);        Moriya, H., K. Nagano and H. Niitsuma: “Precise source location of AE doublets by spectral matrix analysis of the triaxial hodogram”, Geophysics, 59, 36-45 (1994);        Pearson, C: “The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks”, Journ. Of Geophys Res. 86 (B9), 7855-7864 (1981);        Phillips W. S., T. D. Fairbanks, J. T. Rutledge, D. W. Anderson: Induced microearthquake patterns and oil-producing fracture systems in the Austin chalk. Tectonophysics, 289, pp. 153-169(1998); and        Rutledge, J. T. and Phillips, W. S. (2003): Hydraulic stimulation of natural fractures as revealed by induced microearthquakes, Carthage Cotton Valley gas field, east Texas. Geophysics, 68, 441-452.        
Known methods for representing and inverting measured signals for moment tensors are described for example in the published US patent application US 2005/0190649 A1 to Eisner and in publications cited in relation to this application and include:                Riedesel, M. A., and T. H. Jordan, Display and assessment of seismic moment tensors, Bull. Seism. Soc. Am., 79, 85-100 (1989).        Dahm, T. et al Automated moment tensor inversion to estimate source mechanisms of hydraulicallly induced micro-seismicity in salt rock, Tectonophysics 306, 1-17(1999);        Nolen-Hoeksema, R. C. and L. J. Ruff, Moment tensor inversion of microseismic events from hydrofractures, SEG 1999 expanded abstracts (1999);        Nolen-Hoeksema, R. C., and L. J. Ruff, Moment tensor inversion of microseism from the B-sand propped hydrofracture, M-site, Colorado. Tectonophysics, 336, 163-181(2001);        Jechumtálová, Z., and J. {hacek over (S)}ilený, Point-source parameters from noisy waveforms: error estimate by Monte Carlo simulation. Pure Appl. Geophys., 158,1639-1654(2001);        Vavry{hacek over (c)}uk, V., Inversion for parameters of tensile earthquakes. J. Geophys. Res., 106 (B8), 16339-16355 (2001);        Jechumtálová, Z., and J. {hacek over (S)}ílený, Amplitude ratios for complete moment tensor retrieval. Geophys. Res. Lett., 32, L22303 (2005); and        Vavry{hacek over (c)}uk, V. On the retrieval of moment tensors from borehole data. Geophysical Prospecting, 55 (3), 381-391(2007).        
The present invention seeks to improve the accuracy of source related parameters derived from microseismic signals, particularly for monitoring fracturing operations.